Among wireless communication systems, wireless local loop (WLL) or fixed wireless access (FWA) are known for wireless subscriber access systems. As shown in FIG. 8, via wireless communication, a base station 1 that a telecommunication enterprise places, communicates with a plurality of subscriber stations 2 at residential sites. Each of the subscriber stations 2 is connected to a local network (LAN) 3 for data communication. Furthermore, the base station 1 is connected to other subscriber LANs for data communication via back-bone networks 4 such as public networks that are connected to the base station 1. The base station 1 has a plurality of subscriber stations 2, and the multi-directional wireless facility is called point-to-multi points (P-MP) SYSTEM. By distributing a plurality of the above described base stations 1, a predetermined area is covered for providing wireless service to many subscriber stations 2. In order to efficiently use the frequency resource, the same frequency must be repeatedly used for the above communication such as automobile phones and hand-held wireless phones. However, to avoid interference, the same frequency has not been used between adjacent base stations.
The above P-MP systems generally utilize high frequencies such as semi-millimeter waves or a millimeter waves. Since wireless communication is established via high efficiency antennas, the standards have been determined by ARIB STD-T59. According to the standard, the communication frequency utilizes the 26 GH2-band or the 38 GH2 band and the high efficiency antenna over 20 dBi at a subscriber station. For the above reasons, directional antennas such as parabolic antennas and sector antennas are used at the base station 1 to use the repeated frequency for implementing the high efficiency. By placing a plurality of parabolic antennas at the base stations and directing them at a predetermined angle with each other, the horizontal directions are divided into a plurality of sectors for implementing the repeated frequency use for the high efficiency. As shown in FIG. 9, the sector antenna includes a circular disk antenna base plate 10, which is divided into four sectors A through D by an insulation plate 11 and an antenna element 12 at each of the four sectors A through D for antenna directionality. The sectors A through D divide the horizontal directions into four for the repeated frequency use.
In the subscriber wireless access system, the interference between the sectors due to the repeated frequency use is avoided by the directional antenna 7 at the mobile subscriber station 2 that are included in each sector of the base station 1. Japanese Patent Publication Hei 10-42352 disclosed a method of forming cells in a wireless communication system in which a large area is covered by a plurality of communication cells that are created by a plurality of distributed base stations. The communication service area is formed around the base station that is located at an approximate center of the communication service area. According to the above disclosure, as shown in FIG. 10, each base station B1 through B4 has a communication service area as indicated by a hexagonal, and a separate frequency or channel for each antenna or sector is indicated by F1 through F3. In The above prior art, a common frequency is used for m sectors in a cell so that interference between sectors is avoided. Using the above configuration, a subscriber station C1 belongs to a sector that utilizes the channel F2 for the base station B2. The transmission paths of a subscriber station C1 are indicated by arrows. The nearest base station B4 has the highest probability for giving or receiving interference with respect to the subscriber station C1. The above described interference level due to the same channel over one cell does not cause a practical problem to maintain a predetermined communication quality. However, the actual base station placement is not often according to a plan or an advanced design due to surrounding environment and the procurement of the installation location. Because of the deviated actual placement location of the base stations, interference may occur between the cells. FIG. 11 shows an example of the placement deviations of the base station B3 from an original installation location. The subscriber station C2 that belongs to a sector using the channel F2 of the base station B3 possibly has interference with a sector using the channel F2 of the adjacent base station B2. With the hexagonal cell configuration, at least three cells are necessary for cell expansion. At the same time, even if four channels are available, it is difficult to use the four channels in an efficient manner. For the above reasons, it is desired to provide a wireless communication system having an implementable cell placement with at least two channels for preventing interference between adjacent cells.